The present invention relates to an electronic flash unit. Particularly, it relates to an electronic flash unit for use in a photographic process wherein the flashtube is ignited upon operation of a synchronous switch coupled to the shutter of the camera and wherein exposure control means are furnished which terminate the flash when a predetermined exposure of the film has been reached.
More particularly, the present invention relates to flash units wherein the exposure control circuit is activated substantially simultaneously with the initiation of the flash, so that the effect of external flashes or of light which falls upon the exposure control circuit prior to the initiation of the flash is eliminated. One field of application of such an electronic flash unit is in conjunction with a camera. When so used, it operates as follows: The light from the light flash is reflected by the object to be photographed. The reflected light is applied to the film through the optical system of a camera. Simultaneously, it is applied to a photoelectric transducing element which is a part of the exposure control circuit. The photoelectric transducing element furnishes an electric current which corresponds to the light impinging thereon.
This current is integrated, for example by a capacitor, the voltage across the capacitor then constituting a measure of the total quantity of reflected light. If the voltage across the capacitor has reached a predetermined value which corresponds to the optimum film exposure, the circuit acts to terminate the flash.
In such equipment it is of course obvious that if the exposure control means are operative immediately upon energization of the equipment and prior to the time that the flash is to be initiated, then the exposure control means will react to integrate ambient light or light emanating from flashes other than the flash of the particular flash unit. The integration of the current corresponding to this light causes an initial voltage to appear across the capacitor which serves to furnish the terminating signal. Of course an incorrect exposure will then result, since the light furnished by the flash of the unit will be less than that required for optimum exposure. Particularly large errors can be introduced by external flashes which, when directed to impinge on the photoelectric element in the exposure control circuit will have a far greater effect then even the flash of the particular unit. When the circuit terminating the flash contains a quench tube which is connected in parallel with the flashtube and short-circuits same after the voltage across the above-mentioned capacitor has reached the proper voltage for exposure, then such a foreign flash can cause an ignition of the quench tube and a resulting discharge of the storage capacitor, so that the equipment will not be ready to furnish a flash.
Various methods are known wherein the exposure control circuit is not activated until after the ignition pulse has been furnished for the flash tube or until after the flash has actually occurred. Thus the voltage across a capacitor which constitutes the operating voltage for the exposure control circuit is derived either from the flow of ignition current or from the flow of current through the flashtube itself after ignition, or, alternatively, a light sensitive element can be used which, upon receipt of light from the flashtube connects the exposure control circuit to the proper operating voltage.
The above circuits have in common that the light resulting from the beginning of the flash may not be properly evaluated by the exposure control circuit, since this circuit may not be properly operative at that time.